The cyclodiphosph(III/V)azane monoselenide [Bu-t(H)N(Se)P(mu-(NBu)-Bu-t)(2)PN(H)Bu-t] (6, H(2)cdppSe where cdpp = cyclodiphosphazane) is obtained in quantitative yield from the comproportionation reaction of equimolar amounts of H(2)cdpp (7) and H(2)cdppSe(2) (3) in toluene at 85 degreesC. The X-ray structures of 3 and 6 reveal that both the monoselenide and the diselenide adopt a cis,endo,exo arrangement of the (BuNH)-Bu-t groups. Metalation of 6 with 1 equiv of (KOBu)-Bu-t or 2 equiv of KCH2Ph in THF produces {(THF)K[HcdppSe]}(2) (8) or {[(THF)(2)K](2)[cdppSe]}(2) (9), respectively. The initial deprotonation involves the tBuNH group attached to the P(V) center in 6. In the dimeric structure of 8, both of the monoanionic ligands are bis-N,Se-chelated to the two K+ cations to give a distorted K2N2Se2 octahedron. In the centrosymmetric dimer 9, the dianionic ligands adopt two different coordination modes to the bis-solvated K+ ions, viz., tridentate N,N',N" coordination and N,Se-chelation involving both exo- and endocyclic nitrogen atoms. The dimer is linked through K-Se interactions. The reaction of 2 equiv of 8 with NiCl2(PMe3)(2) in THF produces [Ni(HcdppSe)(2)], which has a distorted tetrahedral structure and exhibits anomalous H-1, C-13, and P-31 NMR chemical shifts owing to the proximity of the paramagnetic Ni(II) center.